Re: [PATCH v4 2/7] landlock: Add IOCTL access right

["Mickaël Salaün" <mic@digikod.net>]

On Fri, Nov 03, 2023 at 04:57:12PM +0100, Günther Noack wrote:
> Introduces the LANDLOCK_ACCESS_FS_IOCTL access right
> and increments the Landlock ABI version to 5.
> 
> Like the truncate right, these rights are associated with a file
> descriptor at the time of open(2), and get respected even when the
> file descriptor is used outside of the thread which it was originally
> opened in.
> 
> A newly enabled Landlock policy therefore does not apply to file
> descriptors which are already open.
> 
> If the LANDLOCK_ACCESS_FS_IOCTL right is handled, only a small number
> of safe IOCTL commands will be permitted on newly opened files.  The
> permitted IOCTLs can be configured through the ruleset in limited ways
> now.  (See documentation for details.)
> 
> Noteworthy scenarios which require special attention:
> 
> TTY devices support IOCTLs like TIOCSTI and TIOCLINUX, which can be
> used to control shell processes on the same terminal which run at
> different privilege levels, which may make it possible to escape a
> sandbox.  Because stdin, stdout and stderr are normally inherited
> rather than newly opened, IOCTLs are usually permitted on them even
> after the Landlock policy is enforced.
> 
> Some legitimate file system features, like setting up fscrypt, are
> exposed as IOCTL commands on regular files and directories -- users of
> Landlock are advised to double check that the sandboxed process does
> not need to invoke these IOCTLs.
> 
> Known limitations:
> 
> The LANDLOCK_ACCESS_FS_IOCTL access right is a coarse-grained control
> over IOCTL commands.  Future work will enable a more fine-grained
> access control for IOCTLs.
> 
> In the meantime, Landlock users may use path-based restrictions in
> combination with their knowledge about the file system layout to
> control what IOCTLs can be done.  Mounting file systems with the nodev
> option can help to distinguish regular files and devices, and give
> guarantees about the affected files, which Landlock alone can not give
> yet.
> 
> Signed-off-by: Günther Noack <gnoack@google.com>
> ---
>  include/uapi/linux/landlock.h                | 31 +++++---
>  security/landlock/fs.c                       | 74 ++++++++++++++++++--
>  security/landlock/limits.h                   | 10 ++-
>  security/landlock/ruleset.h                  | 53 +++++++++++++-
>  security/landlock/syscalls.c                 |  6 +-
>  tools/testing/selftests/landlock/base_test.c |  2 +-
>  tools/testing/selftests/landlock/fs_test.c   |  5 +-
>  7 files changed, 161 insertions(+), 20 deletions(-)
> 
> diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
> index 25c8d7677539..6d41c059e910 100644
> --- a/include/uapi/linux/landlock.h
> +++ b/include/uapi/linux/landlock.h
> @@ -128,7 +128,7 @@ struct landlock_net_port_attr {
>   * files and directories.  Files or directories opened before the sandboxing
>   * are not subject to these restrictions.
>   *
> - * A file can only receive these access rights:
> + * The following access rights apply only to files:
>   *
>   * - %LANDLOCK_ACCESS_FS_EXECUTE: Execute a file.
>   * - %LANDLOCK_ACCESS_FS_WRITE_FILE: Open a file with write access. Note that
> @@ -138,12 +138,13 @@ struct landlock_net_port_attr {
>   * - %LANDLOCK_ACCESS_FS_READ_FILE: Open a file with read access.
>   * - %LANDLOCK_ACCESS_FS_TRUNCATE: Truncate a file with :manpage:`truncate(2)`,
>   *   :manpage:`ftruncate(2)`, :manpage:`creat(2)`, or :manpage:`open(2)` with
> - *   ``O_TRUNC``. Whether an opened file can be truncated with
> - *   :manpage:`ftruncate(2)` is determined during :manpage:`open(2)`, in the
> - *   same way as read and write permissions are checked during
> - *   :manpage:`open(2)` using %LANDLOCK_ACCESS_FS_READ_FILE and
> - *   %LANDLOCK_ACCESS_FS_WRITE_FILE. This access right is available since the
> - *   third version of the Landlock ABI.
> + *   ``O_TRUNC``.  This access right is available since the third version of the
> + *   Landlock ABI.
> + *
> + * Whether an opened file can be truncated with :manpage:`ftruncate(2)` or used
> + * with `ioctl(2)` is determined during :manpage:`open(2)`, in the same way as
> + * read and write permissions are checked during :manpage:`open(2)` using
> + * %LANDLOCK_ACCESS_FS_READ_FILE and %LANDLOCK_ACCESS_FS_WRITE_FILE.
>   *
>   * A directory can receive access rights related to files or directories.  The
>   * following access right is applied to the directory itself, and the
> @@ -198,13 +199,26 @@ struct landlock_net_port_attr {
>   *   If multiple requirements are not met, the ``EACCES`` error code takes
>   *   precedence over ``EXDEV``.
>   *
> + * The following access right applies both to files and directories:
> + *
> + * - %LANDLOCK_ACCESS_FS_IOCTL: Invoke :manpage:`ioctl(2)` commands on an opened
> + *   file or directory.
> + *
> + *   This access right applies to all :manpage:`ioctl(2)` commands, except of
> + *   ``FIOCLEX``, ``FIONCLEX``, ``FIONBIO``, ``FIOASYNC`` and ``FIONREAD``.
> + *   These commands continue to be invokable independent of the
> + *   %LANDLOCK_ACCESS_FS_IOCTL access right.
> + *
> + *   This access right is available since the fourth version of the Landlock

It is now the fifth version. Same for the documentation.

> + *   ABI.
> + *
>   * .. warning::
>   *
>   *   It is currently not possible to restrict some file-related actions
>   *   accessible through these syscall families: :manpage:`chdir(2)`,
>   *   :manpage:`stat(2)`, :manpage:`flock(2)`, :manpage:`chmod(2)`,
>   *   :manpage:`chown(2)`, :manpage:`setxattr(2)`, :manpage:`utime(2)`,
> - *   :manpage:`ioctl(2)`, :manpage:`fcntl(2)`, :manpage:`access(2)`.
> + *   :manpage:`fcntl(2)`, :manpage:`access(2)`.
>   *   Future Landlock evolutions will enable to restrict them.
>   */
>  /* clang-format off */
> @@ -223,6 +237,7 @@ struct landlock_net_port_attr {
>  #define LANDLOCK_ACCESS_FS_MAKE_SYM			(1ULL << 12)
>  #define LANDLOCK_ACCESS_FS_REFER			(1ULL << 13)
>  #define LANDLOCK_ACCESS_FS_TRUNCATE			(1ULL << 14)
> +#define LANDLOCK_ACCESS_FS_IOCTL			(1ULL << 15)
>  /* clang-format on */
>  
>  /**
> diff --git a/security/landlock/fs.c b/security/landlock/fs.c
> index bc7c126deea2..aa54970c235f 100644
> --- a/security/landlock/fs.c
> +++ b/security/landlock/fs.c
> @@ -7,12 +7,14 @@
>   * Copyright © 2021-2022 Microsoft Corporation
>   */
>  
> +#include <asm/ioctls.h>
>  #include <linux/atomic.h>
>  #include <linux/bitops.h>
>  #include <linux/bits.h>
>  #include <linux/compiler_types.h>
>  #include <linux/dcache.h>
>  #include <linux/err.h>
> +#include <linux/falloc.h>
>  #include <linux/fs.h>
>  #include <linux/init.h>
>  #include <linux/kernel.h>
> @@ -28,6 +30,7 @@
>  #include <linux/types.h>
>  #include <linux/wait_bit.h>
>  #include <linux/workqueue.h>
> +#include <uapi/linux/fiemap.h>
>  #include <uapi/linux/landlock.h>
>  
>  #include "common.h"
> @@ -147,7 +150,8 @@ static struct landlock_object *get_inode_object(struct inode *const inode)
>  	LANDLOCK_ACCESS_FS_EXECUTE | \
>  	LANDLOCK_ACCESS_FS_WRITE_FILE | \
>  	LANDLOCK_ACCESS_FS_READ_FILE | \
> -	LANDLOCK_ACCESS_FS_TRUNCATE)
> +	LANDLOCK_ACCESS_FS_TRUNCATE | \
> +	LANDLOCK_ACCESS_FS_IOCTL)
>  /* clang-format on */
>  
>  /*
> @@ -157,6 +161,7 @@ int landlock_append_fs_rule(struct landlock_ruleset *const ruleset,
>  			    const struct path *const path,
>  			    access_mask_t access_rights)
>  {
> +	access_mask_t handled;
>  	int err;
>  	struct landlock_id id = {
>  		.type = LANDLOCK_KEY_INODE,
> @@ -169,9 +174,11 @@ int landlock_append_fs_rule(struct landlock_ruleset *const ruleset,
>  	if (WARN_ON_ONCE(ruleset->num_layers != 1))
>  		return -EINVAL;
>  
> +	handled = landlock_get_fs_access_mask(ruleset, 0);
> +	/* Expands the synthetic IOCTL groups. */
> +	access_rights |= expand_all_ioctl(handled, access_rights);
>  	/* Transforms relative access rights to absolute ones. */
> -	access_rights |= LANDLOCK_MASK_ACCESS_FS &
> -			 ~landlock_get_fs_access_mask(ruleset, 0);
> +	access_rights |= LANDLOCK_MASK_ACCESS_FS & ~handled;
>  	id.key.object = get_inode_object(d_backing_inode(path->dentry));
>  	if (IS_ERR(id.key.object))
>  		return PTR_ERR(id.key.object);
> @@ -1123,7 +1130,9 @@ static int hook_file_open(struct file *const file)
>  {
>  	layer_mask_t layer_masks[LANDLOCK_NUM_ACCESS_FS] = {};
>  	access_mask_t open_access_request, full_access_request, allowed_access;
> -	const access_mask_t optional_access = LANDLOCK_ACCESS_FS_TRUNCATE;
> +	const access_mask_t optional_access =
> +		LANDLOCK_ACCESS_FS_TRUNCATE | LANDLOCK_ACCESS_FS_IOCTL |
> +		IOCTL_CMD_G1 | IOCTL_CMD_G2 | IOCTL_CMD_G3 | IOCTL_CMD_G4;

I think it would be more future-proof to declare a new global const and
use it here for optional_access:

static const access_mask_t ioctl_groups =
	IOCTL_CMD_G1 |
	IOCTL_CMD_G2 |
	IOCTL_CMD_G3 |
	IOCTL_CMD_G4;

>  	const struct landlock_ruleset *const dom = get_current_fs_domain();
>  
>  	if (!dom)
> @@ -1196,6 +1205,62 @@ static int hook_file_truncate(struct file *const file)
>  	return -EACCES;
>  }
>  
> +static access_mask_t required_ioctl_access(unsigned int cmd)
> +{
> +	switch (cmd) {
> +	case FIOQSIZE:
> +		return IOCTL_CMD_G1;
> +	case FS_IOC_FIEMAP:
> +	case FIBMAP:
> +	case FIGETBSZ:
> +		return IOCTL_CMD_G2;
> +	case FIONREAD:
> +	case FIDEDUPERANGE:
> +		return IOCTL_CMD_G3;
> +	case FICLONE:
> +	case FICLONERANGE:
> +	case FS_IOC_RESVSP:
> +	case FS_IOC_RESVSP64:
> +	case FS_IOC_UNRESVSP:
> +	case FS_IOC_UNRESVSP64:
> +	case FS_IOC_ZERO_RANGE:
> +		return IOCTL_CMD_G4;
> +	case FIOCLEX:
> +	case FIONCLEX:
> +	case FIONBIO:
> +	case FIOASYNC:
> +		/*
> +		 * FIOCLEX, FIONCLEX, FIONBIO and FIOASYNC manipulate the FD's
> +		 * close-on-exec and the file's buffered-IO and async flags.
> +		 * These operations are also available through fcntl(2),
> +		 * and are unconditionally permitted in Landlock.
> +		 */
> +		return 0;

You can move this block at the top.

> +	default:
> +		/*
> +		 * Other commands are guarded by the catch-all access right.
> +		 */
> +		return LANDLOCK_ACCESS_FS_IOCTL;
> +	}
> +}
> +
> +static int hook_file_ioctl(struct file *file, unsigned int cmd,
> +			   unsigned long arg)
> +{
> +	access_mask_t required_access = required_ioctl_access(cmd);
> +	access_mask_t allowed_access = landlock_file(file)->allowed_access;

You can use const for these variables.

> +
> +	/*
> +	 * It is the access rights at the time of opening the file which
> +	 * determine whether ioctl can be used on the opened file later.
> +	 *
> +	 * The access right is attached to the opened file in hook_file_open().
> +	 */
> +	if ((allowed_access & required_access) == required_access)
> +		return 0;

Please add a new line.

> +	return -EACCES;
> +}
> +
>  static struct security_hook_list landlock_hooks[] __ro_after_init = {
>  	LSM_HOOK_INIT(inode_free_security, hook_inode_free_security),
>  
> @@ -1218,6 +1283,7 @@ static struct security_hook_list landlock_hooks[] __ro_after_init = {
>  	LSM_HOOK_INIT(file_alloc_security, hook_file_alloc_security),
>  	LSM_HOOK_INIT(file_open, hook_file_open),
>  	LSM_HOOK_INIT(file_truncate, hook_file_truncate),
> +	LSM_HOOK_INIT(file_ioctl, hook_file_ioctl),
>  };
>  
>  __init void landlock_add_fs_hooks(void)
> diff --git a/security/landlock/limits.h b/security/landlock/limits.h
> index 93c9c6f91556..d0a95169ba3f 100644
> --- a/security/landlock/limits.h
> +++ b/security/landlock/limits.h
> @@ -18,7 +18,15 @@
>  #define LANDLOCK_MAX_NUM_LAYERS		16
>  #define LANDLOCK_MAX_NUM_RULES		U32_MAX
>  
> -#define LANDLOCK_LAST_ACCESS_FS		LANDLOCK_ACCESS_FS_TRUNCATE
> +#define LANDLOCK_LAST_PUBLIC_ACCESS_FS	LANDLOCK_ACCESS_FS_IOCTL
> +#define LANDLOCK_MASK_PUBLIC_ACCESS_FS	((LANDLOCK_LAST_PUBLIC_ACCESS_FS << 1) - 1)
> +

Please add a comment to explain that the semantic of these synthetic
access rights is defined by the required_ioctl_access() helper.

> +#define IOCTL_CMD_G1			(LANDLOCK_LAST_PUBLIC_ACCESS_FS << 1)
> +#define IOCTL_CMD_G2			(LANDLOCK_LAST_PUBLIC_ACCESS_FS << 2)
> +#define IOCTL_CMD_G3			(LANDLOCK_LAST_PUBLIC_ACCESS_FS << 3)
> +#define IOCTL_CMD_G4			(LANDLOCK_LAST_PUBLIC_ACCESS_FS << 4)
> +
> +#define LANDLOCK_LAST_ACCESS_FS		IOCTL_CMD_G4
>  #define LANDLOCK_MASK_ACCESS_FS		((LANDLOCK_LAST_ACCESS_FS << 1) - 1)
>  #define LANDLOCK_NUM_ACCESS_FS		__const_hweight64(LANDLOCK_MASK_ACCESS_FS)
>  #define LANDLOCK_SHIFT_ACCESS_FS	0
> diff --git a/security/landlock/ruleset.h b/security/landlock/ruleset.h
> index c7f1526784fd..58d96aff3980 100644
> --- a/security/landlock/ruleset.h
> +++ b/security/landlock/ruleset.h
> @@ -30,7 +30,7 @@
>  	LANDLOCK_ACCESS_FS_REFER)
>  /* clang-format on */
>  
> -typedef u16 access_mask_t;
> +typedef u32 access_mask_t;
>  /* Makes sure all filesystem access rights can be stored. */
>  static_assert(BITS_PER_TYPE(access_mask_t) >= LANDLOCK_NUM_ACCESS_FS);
>  /* Makes sure all network access rights can be stored. */
> @@ -256,6 +256,54 @@ static inline void landlock_get_ruleset(struct landlock_ruleset *const ruleset)
>  		refcount_inc(&ruleset->usage);
>  }
>  
> +/**
> + * expand_ioctl - return the dst flags from either the src flag or the

Return...

> + * LANDLOCK_ACCESS_FS_IOCTL flag, depending on whether the
> + * LANDLOCK_ACCESS_FS_IOCTL and src access rights are handled or not.
> + *
> + * @handled: Handled access rights
> + * @access:  The access mask to copy values from
> + * @src:     A single access right to copy from in @access.
> + * @dst:     One or more access rights to copy to

Please don't add extra spaces, that would avoid to shift all the lines
if we get another name which is longer.

> + *
> + * Returns:
> + * @dst, or 0
> + */
> +static inline access_mask_t expand_ioctl(access_mask_t handled,
> +					 access_mask_t access,
> +					 access_mask_t src, access_mask_t dst)
> +{
> +	if (!(handled & LANDLOCK_ACCESS_FS_IOCTL))
> +		return 0;
> +
> +	access_mask_t copy_from = (handled & src) ? src :
> +						    LANDLOCK_ACCESS_FS_IOCTL;
> +	if (access & copy_from)
> +		return dst;

Please add a newline after return.

> +	return 0;
> +}
> +
> +/**
> + * Returns @access with the synthetic IOCTL group flags enabled if necessary.

I think htmldocs will print a warning with this format.

> + *
> + * @handled: Handled FS access rights.
> + * @access:  FS access rights to expand.
> + *
> + * Returns:
> + * @access expanded by the necessary flags for the synthetic IOCTL access rights.
> + */